Gadgetory

now of course we all recognize the MPG in mpge that's the easy part but that little lowercase e at the end that's actually surprisingly powerful let's break down the acronym first of all mpg e stands for miles per gallon of gasoline equivalent now the equivalent part is what we're talking about energy in a gallon of gas as opposed to the volume of a gallon of gas which is what mpg basically refers to here's how they work that out numerically the EPA says that a gallon of gas has 115,000 BTUs or British thermal units of potential energy in it and that they say is equivalent to thirty three point seven kilowatt hours of stored electricity kilowatt hours is typically how you measure the capacity of a battery now the math isn't simple as you can imagine and this is part of how they actually pull that equation together but let's take a look at how the cars that use MPG are sorted out a battery electric car can have mpge like a Nissan Leaf or a Tesla Model S all you do is plug them in that's their only source of power then you've got plug-in hybrids those are vehicles that have a gas engine and a bigger battery and can still run in a blended or separate modes you've also got hydrogen fuel cell car which is also electric but using a different source to generate the volts what you won't find an MPG rating on is a conventional hybrid because the epa has just deemed those to be too conventional now if you look at the stickers it's an interesting story here's a battery electric car all that has is an mpge number because it doesn't run in any other mode there's no gas involved go to a plug-in hybrid and it gets very busy here's your mpg II on the Left which shows the pure electric running on the right here's the blended system number that shows the fact that it also can run on gas that's your mpg number what I also want you to notice is what's in the fine print on these new stickers look underneath either the mpg or the MPGe and you now find a european-style gallons per hundred miles or over here kilowatt hours in hundred miles this is not just semantics if you do this you can avoid what's called the mpg illusion which basically says that if you measure a car that improves from 10 to let's say 15 mpg that is not the same as a car that improves from let's say 50 to 55 you might think those are equivalent gains in fact the lower number improvement is vastly more significant than the higher number improvement of the car that started at 50 but if you measure in energy per hundred miles its linear the distortion is taken out mathematically and of course the gas engine car doesn't get MPGe at all you'll see your traditional number there on the sticker still because all it does is run on gas more car tech demystified right now at CNET on cars calm click on car tech 101